The invention relates to formation of electrical contacts between an electron device and a substrate and, more particularly, to in-situ formation of conductive diamond film regions for crystalline particle interconnects with an electron device.
Particle interconnects are conductive particles embedded in conductive traces on an insulative or semiconductive substrate for scratching the surface of an electron device, usually a semiconductor chip, for the purpose of making electrical contact upon mechanical compression of the chip and substrate. Diamond particles screened to sizes between 10 and 125 microns are electrically plated onto a substrate in patterns defined by standard photoresist processes used in the semiconductor industry. The patterns are designed to match leads or pins or contacts of semiconductor chips. Upon pressing a chip onto a substrate having diamond particles embedded in patterns of corresponding traces, electrical contact is made along the traces through the leads, pins or contacts of the chip.
In another approach, diamond particles are sifted down to 100 grit material which is trapped in a screen having pores of similar dimensions. The screen is placed on a metalized surface, rolled onto the surface, then plated over to keep the diamond particles in place.
U.S. Pat. No. 5,083,697 to Difrancesco teaches a method of joining two metal surfaces using metallized particles having a hardness greater than the metals to be joined. The patent teaches that high local stress causes elastoplastic deformation which promotes formation of diffusion bonds between the materials.
Exatron Corporation of San Jose, Calif. has devised a micro-miniature bed-of-nails formed by diamond particles in an electroplate matrix on a substrate. The particles make electrical connection between contacts of a chip and the substrate by penetrating the oxides on the contacts by scratching through surface oxide layers. The particles are small enough so that the contacts are not damaged, but large enough so that a low resistance path is established. The prior art approach is shown in FIG. 1. A support 11 carries an insulative layer 13, such as a ceramic, polyimide or Kapton layer which is only a few thousandths of an inch thick. A thin conductive layer 15, thinner than insulative layer 13, is electroplated onto the insulative layer. The conductive layer is a matrix, such as nickel, for holding small diamond particles 17, some of which are shown protruding from the conductive layer 15. An electrical lead 19 is shown to have an oxide coating 21. This oxide coating is penetrated or scratched by diamond particles 17 when lead 19 is pushed down onto conductive layer 15, thereby establishing electrical contact between lead 19 and conductive layer 15.
A typical dimension for the width or thickness of electrical lead 19 with oxide coating 21 is indicated by the letter xe2x80x9cAxe2x80x9d and is about 0.010 inches. The height of diamond particles 17 above conductive layer 15 is indicated by the letter xe2x80x9cBxe2x80x9d and is about 0.001 inches. The combined height of diamond particles 17, conductive layer 15 and insulative layer 13 is indicated by the letter xe2x80x9cCxe2x80x9d and is about 0.004 inches.
While particle interconnect technology looks promising, the formation of diamond particle electroplate slurries is difficult because of non-uniformities in the mixture. Diamond particles tend to clump together due to electrostatic forces and mixing difficulties. If the electroplate slurry is not uniform the deposited diamond film will be non-uniform. An object of the invention has been to devise a method of forming uniform diamond particle conductive traces for electrical interconnects.
The above object has been achieved by in-situ formation of a conductive polycrystalline diamond layer on a substrate. The layer has sharp crystalline diamond facets projecting therefrom and is formed so that portions of the layer are in appropriate places for making electrical contact with a semiconductor device which will be brought into pressure contact with the layer. Rather than providing a slurry of electroplate material with diamond particles mixed therein, the present invention provides a single conductive diamond film layer, with portions of the film having sharp diamond crystal facets extending from the film.
In one embodiment, a substrate or base capable of withstanding 800xc2x0 C. without deformation is placed in a heated diamond film gas phase deposition reactor. Such reactors have been known for a long time and are commercially available. A diamond film is formed over the whole surface of the substrate by introducing molecular hydrogen, a carbon bearing gas, such as an alkane, to be cracked within the reactor to form molecular carbon and a dopant source, all at a temperature conducive to the deposition of a polycrystalline diamond layer on the substrate base. The polycrystalline diamond layer, having exposed sharp facets, is etched down to the substrate base to define a desired pattern of contact zones and traces of polycrystalline diamond particles for use as interconnects. The dopant source provides sufficient conductivity so that the diamond particles themselves and the film may form conductive traces for electrical contact with a device, particularly where the substrate does not have previously formed metal traces.
The described in-situ formation method has no matrix which holds the diamond particles. Rather, the diamond particles are in a continuous diamond film with crystalline facets projecting from the film. This is a distinct change relative to the prior art which has taken the approach of embedding diamond particles in various matrices for different applications. A diamond film has the distinct advantage of uniformity because mixing of particles within the matrix is not necessary. Diamond crystal facets appear randomly, but with sufficient density that good electrical contact is assured.
In another embodiment, a substrate is roughened by abrading with diamond grit to remove oxides or other films. The substrate is placed in a diamond film reactor as above. A diamond film is formed in-situ on the substrate by introducing molecular hydrogen, a carbon bearing gas to be cracked within the reactor to form molecular carbon and a dopant source, all at a temperature conducive to the deposition of a polycrystalline diamond layer on the substrate base. The diamond layer is etched where no contacts are wanted, leaving polycrystalline diamond film on the contact regions.